IRoebling jfuuD 



THE kampomp:ter, a new instrument of 



EXTREME SENSITIVENESS FOR 

 MEASURING RADIATION 



By C. G. abbot 



Secretary, Sinitlisoiiian Institution 



About 25 years ago, as I was using a reflecting galvanometer by 

 White of Glasgow, I noticed that when sunlight fell on its mirror 

 a small deflection occurred without the flow of electric current. I 

 found that this happened because the mirror was fastened to one 

 of the groups of magnets of the suspension instead of lying between 

 the coils as is now more usual. The system was in fact slightly twisted 

 by the warmth of tiie sun ray. It occurred to me that if an astatic 

 suspended system was purjx)sely designed to be deformed by radia- 

 tion, perhaps it might give large deflections with feeble rays. I con- 

 structed such an instrument about March, 1908, and tested it a few 

 months later in the presence of Doctor Hale and Doctor Adams on 

 Mount Wilson. It did indeed show high sensitiveness to radiation 

 impulses but had too large a moment of inertia and a very long 

 period of swing. I never used it for serious work. 



In June, 1932, being again on Mount Wilson, and in need of a 

 quick-acting radiation-measuring instrument of the highest sensi- 

 bility, beyond what I could hope to get with the bolometer or even 

 the radiometer, my thought recurred to this old instrument. It 

 occurred to me that the two groups of magnets of the astatic system 

 could be supported upon a stem made of two close curls of thin 

 metallic ribbon, the two curls being of opposite senses to avoid 

 distortion of the parallelism of the upper and lower magnet groups 

 due to changes of surrounding temperature. I believed that when 

 radiation should fall on one only of the two curls, the parallelism 

 would be distorted and a tendency to rotate the system through 90° 

 would ensue. Doctor Anderson, who encouraged me in this idea, 

 suggested the obvious advantage of making the two curls of bime- 

 tallic strips. I constructed such an instrument, and found it to exceed 

 my expectations as regards sensitiveness and satisfactory behavior. 



Smithsonian Miscellaneous Collections, Vol. 89, No. 3 



